Back-End / API

The Back-End is the heart of the application.
The business logic and the data are managed here.
Depending many factors like the stage of the project, the complexity
of the business logic, the way the application is deployed,
the size of the team working on the project
and many other factors, the most appropriate language can be chosen:

statically typed language (like Java, C# or C++),

dynamically typed language (like Python or JavaScript)

And the most appropriate architecture and technology can be also chosen.
An Application Programming Interface (API) can be also deployed to
interact programmatically with the Back-End, facilitating the
integration with other systems.